The enlargement of the European Union (EU) in 2004, or the “fifth” enlargement, when Latvia also became a member, involved quite possibly the most ambitious transformation of legal thinking, the legal system and public administration throughout the recent history of Europe. The formal framework of the enlargement consisted of relatively standard EU association agreements. However, the internal political and legal processes in the candidate countries, including Latvia, were not considered to be unequivocally stable. Swift and irrevocable progress in implementing EU law was therefore significant. The process was complicated by the fact that the presence and application of international agreements in Latvia was not particularly widespread at the time. The complicated legislative procedures, the small number of professionals permanently involved in this work and politicised debates also significantly slowed the process of law-making, whose aim, in turn, was to form the basis for the stable transformation of the Latvian legal system. This Chapter highlights the most important political and organisational challenges of the EU’s decade of integration (1995–2005) and their solutions, the individual challenges connected to transposition of EU legislation and, most significantly, the impact of integration on the transformation of legal thinking and case law in Latvia.